Many techniques and systems have been utilized to measure atmospheric or clear air turbulence. Generally, from analyzing a model of the atmosphere, some variable or state of the atmosphere that is affected by the turbulence is detected. Known prior art systems include passive and active acoustics, optical stellar scintillation detection, microwave scintillation of radio, star and satellite beacons, infrared and microwave backscatter, tropospheric bistatic radio scatter, and ultra sensitive radar.
A number of difficulties have arisen in utilizing one or more of the foregoing techniques or systems for airborne applications. For example, excessively large antennas are required if microwave techniques are to be employed and difficulties in achieving adequate range and accuracy of velocity information arise with those techniques which rely on the optical stellar scintillation system mentioned hereinabove. Accordingly, pulsed laser radar apparatus have been developed for the measurement of air turbulence suitable for use in an aircraft and capable of providing highly accurate measurements at a substantial range.
For example, in U.S. Pat. No. 3,856,402, a pulsed laser Doppler radar system is described. As described in the '402 patent, a train of short pulses of radiation, generated by a laser source, typically a CO.sub.2 laser, is developed with the direction of propagation of the pulse being in the direction of flight of the aircraft. The laser pulses are backscattered from the atmospheric aerosol. An optical receiver is arranged to detect the backscattered return. The radiation backscattered by atmospheric particulates is fed to an optical receiver in the aircraft and is Doppler shifted by an amount f.sub.D where f.sub.D equals 2V/.lambda.. V is the velocity component along the direction of the pulse propagation between the aircraft and the air in the instantaneously illuminated volume of air and .lambda. is the wavelength of the laser. The length of the propagated pulse determines the spatial resolution in the atmosphere and also the Doppler shift resolution. The turbulence detection capability of the system is a consequence of the simultaneous measurement of Doppler shift from different regions of the instantaneous pulse volume in the atmosphere whence the turbulence in the air is inferred from the bandwidth of the backscattered signal. The distance to the turbulence is inferred from the roundtrip propagation time.
Precise frequency measurement capability is achieved by beating the scattered radiation with a continuous wave, highly stable laser beam, a process which results in a beat frequency directly proportional to the velocity component. The pulsed laser source is derived from a stable, continuous wave laser source by pulse modulating the output of the laser. This technique assures the presence of a reference beam for the homodyne frequency conversion process employed.
A disadvantage and limitation of the system described in the '402 patent is that a master oscillator laser is used to generate each of the local oscillator beam and the high energy pulses for transmission using a laser amplifier. Such a system suffers from insufficient isolation between the transmitter and local oscillator beams. To achieve the necessary isolation, an elaborate system as described in the '402 patent is required. Another disadvantage and limitation of such a system as described in the '402 patent is that the high gain laser amplifier used to generate the high energy transmitter pulses is highly alignment sensitive and are easily affected by environmental parameters such as vibration and temperature variation.
Other known prior art coherent Doppler laser radars use a highly stable low power continuous wave laser as in injection oscillator. The injection oscillator provides the means for frequency locking of the pulsed high power transmitter laser and the moderate power continuous wave local oscillator laser. A disadvantage and limitation of a system that uses an injection oscillator laser in addition to a local oscillator and transmitter lasers is that such systems are not suitable for many applications because of their complexity and size.
It is therefore highly desirable to use a single pulsed laser source for both the transmission and local oscillator in a laser radar system. A system of this type is described in U.S. Pat. No. 4,447,149. The apparatus described in the '149 patent utilizes a single Q-switched laser to generate both the target signal and a local oscillator signal for use in a heterodyne signal detector. After the laser pulse is generated, the laser unit is maintained at a very low signal output level for substantially the majority of time before the generation of the next laser pulse. This is achieved by designing the Q-switch transmission as a function of time in accordance with the lasing media, resonator parameters and the required pulse shape. During the time the laser is operated a very low signal output level, the output of the laser is utilized as a local oscillator and is mixed with the returning target signal prior to application to the detector unit.
A disadvantage and limitation of the radar apparatus disclosed in the '149 patent is that the laser beam including both the high intensity pulse and trailing tail is incident on a beam splitter so that a portion of the entire optical energy is directed to the target and another portion of the total optical energy of each of the high intensity pulse and trailing tail is used as the local oscillator. However, in optical heterodyne detection, it is desired to use a local oscillator with a large enough optical power to allow for the detector quantum noise limited operation. In the system described in '149, it is not practical to provide sufficient amount of local oscillator power without damaging the detector by the high intensity portion of the pulse. Separating the high intensity portion of the beam from the trailing tail eliminates this problem. In addition, due to the loss of energy in the high intensity pulse delivered to the target, the range of the laser radar apparatus is accordingly limited.